\documentclass[11pt,a4paper]{article}
\usepackage{longtable}
\usepackage{arydshln}

\begin{document}

\renewcommand{\arraystretch}{2.3}
\begin{longtable}{l:l}
	\textbf{Amplitude}                        & \textbf{Amplitude*}    \\ \hline
	$\displaystyle \frac{i}{p^2 + i\epsilon}$ &
	$\qquad \displaystyle -\frac{i}{p^2 - i\epsilon}$                  \\
	$\displaystyle \frac{i}{p^2 + i\epsilon}$ &
	$\qquad \displaystyle -\frac{i}{p^2 - i\epsilon}$                  \\
	\multicolumn{2}{c}{$something$}                                    \\
	\multicolumn{2}{c}{$something$}                                    \\
	$-ie \gamma^\mu$                          & $\qquad ie \gamma^\mu$ \\
	$-ie \gamma^\mu$                          & $\qquad ie \gamma^\mu$ \\
\end{longtable}

\end{document}
